1. Field of the Invention
The field of art to which this invention pertains is the isomerization of glucose into fructose. More specifically the invention relates to an isomerization in which high purity glucose is separated from the isomerization reaction effluent stream comprising glucose and fructose by employing an adsorbent comprising a crystalline aluminosilicate which selectively adsorbs the glucose from the effluent stream, and the high purity glucose extract is recycled to the isomerization reaction stage.
2. Description of the Prior Art
Although both glucose and fructose can be employed in many commerical uses requiring sweeteners, fructose is of particular commercial interest because it is considered to be the most soluble and the sweetest of the sugars. Relative to sucrose having a sweetness of 1.0, fructose has a relative sweetness of about 1.4 while that of glucose is 0.7.
It is well known to the art that glucose isomerase, which is produced by certain kinds of bacteria, may be effectively used in the enzymatic conversion of glucose to fructose. For example, processes that involve such conversion are claimed or disclosed in U.S. Pat. Nos. 3,694,314; 3,813,320; 3,909,354; 4,001,082; 4,008,124; 4,029,546; and 4,059,489. The art further recognizes that to enable extended and repeated use of the enzyme its immobilization is advantageous. For instance, glucose isomerase may be bound (immobilized) to diethylaminoethyl cellulose, natural sponge or ion exchange resin.
It is also known to the art that the continuous production of fructose from a high purity glucose feedstock may be achieved by passing the feedstock through a column packed with a bed of the immobilized enzyme. A typical feedstock is obtained by the degradation of corn starch, i.e. hydrolysis of the starch with a mineral acid to produce corn syrup. The syrup, prior to introduction into the isomerization column, is purified and adjusted to the solids concentration, temperature and pH most effective for the type of enzyme being used.
It is known in the separation art that certain crystalline aluminosilicates can be used in the separation of fructose from a mixture of fructose and glucose. For example, adsorbents comprising crystalline aluminosilicate are used in the method described in U.S. Pat. No. 4,014,711 to separate fructose as a high purity extract from a mixture of sugars in aqueous solution including fructose and glucose. The glucose is contained in the raffinate along with impurities such as oligo-saccharides contained in the feed mixture. The crystalline aluminosilicates may be X, Y or L faujasite or mordenite, and be ion-exchanged with cations selected from the group consisting of alkali metal, alkali earth metal, copper, silver, zinc, cadmium, aluminum, lead, iron and cobalt.
It is desirable to recover in a separation stage the unconverted glucose from the isomerization reaction effluent stream for recycle to the isomerization reactor. However, when the above described known separation process is used for such recovery, the glucose, which is in the raffinate stream, is mixed with oligosaccharides and other impurities that have passed through the isomerization reactor and separation stage. These oligosaccharides and other impurities are therefore recycled back to the isomerization reactor along with the glucose and quickly build up in high concentrations in the system, thus necessitating removal or "blow-down" of a portion of the recycle stream with the resultant loss of the glucose contained in that portion.
We have discovered that an adsorbent comprising an X zeolite containing potassium cations at the exchangeable cationic sites uniquely exhibits adsorptive selectivity for glucose with respect to fructose, thereby making separation of glucose from a mixture comprising glucose and fructose by solid-bed selective adsorption possible, and therefore high purity glucose may be obtained as the extract stream from such mixture. Thus, high purity glucose may be recycled free of undesirable oligosaccharides and other impurities back to the isomerization reactor, thereby eliminating the above described problem relating to the build up of such undesirable materials in the system.